Why is the IELTS examiner confused by this? Because it creates in the mind of the examiner a kind of impossible cartoon image showing two condoms having sex in such a way that the female condom will not become pregnant.
Think of the condom as the agent that does the preventing. Humans can avoid pregnancy through the use of condoms, where the condom is the agent of prevention. So..
Condoms can prevent pregnancy.
Women can avoid pregnancy.